Called when the browser associates the element with a form element, or disassociates the element from a form element.
Called after the form is reset. The element should reset itself to some kind of default state. For elements, this usually involves setting the value property to match the value attribute set in markup (or in the case of a checkbox, setting the checked property to match the checked attribute.
Called in one of two circumstances: When the browser restores the state of the element (for example, after a navigation, or when the browser restarts). The mode argument is "restore" in this case.
When the browser's input-assist features such as form auto filling sets a value. The mode argument is "autocomplete" in this case.
The type of the first argument depends on how the setFormValue() method was called. For more details, see Restoring form state.
Lifecycle callbacks A map, whose keys are the strings "connectedCallback", "disconnectedCallback", "adoptedCallback", "attributeChangedCallback", "formAssociatedCallback", "formDisabledCallback", "formResetCallback", and "formStateRestoreCallback" https://html.spec.whatwg.org/multipage/custom-elements.html#concept-custom-element-definition-lifecycle-callbacks
A form-associated custom element API includes a set of extra lifecycle callbacks to tie in to the form lifecycle. The callbacks are optional: only implement a callback if your element needs to do something at that point in the lifecycle. https://html.spec.whatwg.org/multipage/custom-elements.html#the-elementinternals-interface https://docs.google.com/document/d/1JO8puctCSpW-ZYGU8lF-h4FWRIDQNDVexzHoOQ2iQmY/edit